Turn to pneumatics or direct-drive linear?

Which offers the best bang for the buck: a direct pneumatic solution or an integrated linear motor package?

By Control Design staff

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We've always used servomotors to translate rotary motion to linear motion via slides, ball screws and other mechanical components. A necessary upgrade for better reliability and versatility is at hand, and contrary to a lot of market conversation, a more direct pneumatic solution looks promising because of its integration with electronics, manifolds and other controls.

Most of our customers have compressed air systems. But there are equally promising integrated linear motor packages that might save space and be an almost plug-and-play upgrade that our customers might accept. So far, there doesn't look like there's a big cost differential in either case. We'll get an upgrade either way, but is there a prevailing opinion about best bang for the buck here?

—From November '13 Control Design



Upgrades Usually Electric
Assuming the desired process can be done with either electric or pneumatic solutions with the required repeatability/accuracy and flexibility requirements for future needs, and that any difference in floor space is not a concern, then the remaining considerations would likely be environment, life expectancy, operating noise, air contamination (such as the pneumatics lubrication) and, perhaps most important, energy utilization.

If the environment is a typical inside manufacturing environment with little chance of contamination, your most important considerations between the two technologies are life expectancy of the bearings for the electric solution versus the seals for the pneumatics, and the overall energy utilization to accomplish the job.

The life expectancy of both bearings and seals should be available from the component manufacturer based on some baseline design loading criteria, which you may adjust for each specific application for an even better comparison. If total energy utilization becomes too unrealistic to estimate without actual measurements, it might be easier to estimate the percentage savings of one technology over the other by taking the product of all efficiencies of one technology (say electric) over the other (pneumatic) or vice versa. The total product of the pneumatic efficiencies should include that of the air compressor (even if supplied by the factory independent of the machine), as well as all the other applicable efficiencies in the chain.

It has been my experience that the typical technology change/upgrade has been from pneumatic actuators to electric actuators, not the other way around. But without further knowledge of the specific process or job to be performed and the experience of the machine designer, it is impossible to make a judgment call. Each technology has its niche and, in the end, it is all about risk management of the specific technologies available and their interface with each other with the highest priority given to the process or work to be performed that will generate the best and most energy-efficient machine.

As the environmental and monetary cost of power rise, the designer's goal will be to select the best actuator-drive/control-feedback system for the application with the advantages and disadvantages of all technologies to be used. The potential user savings in power utility payments alone justifies a deeper analysis of each axis for the best design.

Hurley Gill,
senior systems/application engineer

Initial Cost or Operating Cost
The primary cost trade-off when considering an electric solution vs. a pneumatic solution is initial cost vs. operating cost. The initial cost of an electric solution will be higher, but its operating cost will be considerably lower, and it could provide more flexibility and higher throughput. Flexibility is realized in quicker changeover times when running different product, and higher throughput can be realized due to high level of control (position and force) that is inherent in electromechanical actuation. If you have been building your "rotary to linear" solutions yourself, you might be able to save space by selecting an electromechanical actuator specifically designed for compactness and power density. These actuators can be configured for stroke length and a host of options to allow it to easily interface with your machine.  Of course, there is no substitute for a real cost estimate. In applications that we've worked with users on, we have found that electromechanical solutions save hundreds of dollars on operating costs and can often result in a payback of less than a year on the higher initial hardware costs.

James Marek,
business unit director,
Thomson Linear Systems

Precision Matters
I am not entirely certain of what the reader's "direct pneumatic solution" is, which he only briefly mentions. But if this is a servo-pneumatic control system, that would be the "best bang for the buck" here if the load in the application is large/heavy and relatively (on par with a pneumatic cylinder) high speeds are required, but where the application can tolerate precision that might be on the order of 0.1 to 0.01-in. (depends on the load, speed, tuning, overall length, etc.). 

However, if extreme precision, repeatability, flexibility, scalability and reliability are most important, then an integrated linear motor package might be the best solution, since the integrated motor can provide precision down to 0.001 in. or even better, depending on all the applicable variables of the application, and offers optimum positioning flexibility.

But it is important to note that the integrated motor will not provide the thrust and speed performance per bore size of the equivalent pneumatic solution.

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